In many magnetic recording systems, the track density of a magnetic disk is increased in order to increase recording capacity. When tracking density is high, accurate positioning or tracking of a magnetic head on a magnetic recording disk is important. Many types of tracking servo systems have been developed to provide accurate tracking. In many tracking servo systems, a magnetic head reproduces a positioning signal recorded on a magnetic disk and a head actuator controls the magnetic head to position the magnetic head-at the center of the track in accordance with this reproduced positioning signal.
When a magnetic head records a positioning signal on a magnetic disk, a moving mechanism controls the magnetic head to an accurate position. Therefore, the accuracy of the positioning signal, or tracking accuracy, depends on the accuracy of the moving mechanism. However, it is difficult to increase the accuracy of the moving mechanism. In addition, the relatively low accuracy of the moving mechanism prevents the track density from being sufficiently high.
To solve this problem, a magnetic disk having a projection pattern is suggested. The magnetic disk is prepared as follows. A projection pattern is formed on a surface of a non-magnetic substrate by a photolithography and so on, a magnetic layer is put on the surface of the non-magnetic substrate, a magnetization is recorded on the projection of the magnetic layer and an inverse magnetization is recorded on the bump of the magnetic layer. The accuracy of the projection pattern formed by a photolithography determines the accuracy of the positioning signal.
When the positioning signal is recorded on the magnetic layer, the difference between the distance from the magnetic head to the surface of the magnetic layer of the projection and from the magnetic head to the surface of the magnetic layer of the bump is utilized. The magnetic layer of the projection is comparatively near to the magnetic head. Therefore, it is easy to magnetize the magnetic layer of the projection in the direction. But the magnetic layer of the bump is comparatively far from the magnetic head. Therefore, it is difficult to magnetize the magnetic layer of the bump in the direction.
FIGS. 15a and 15b show a method for recording a positioning signal on a magnetic disk. A magnetic disk 1 has a projection pattern (a projection 13 and a bump 14) on a surface. A DC current is impressed on a magnetic head having a magnetic gap G of a comparatively strong magnetic field M.sub.1 reaching a bump 14 of a magnetic disk 1. While the magnetic disk 1 is rotated in the direction of the arrow A, the magnetic head 3a is moved in the direction of the diameter. The magnetization m1 is recorded on the all area, including the projection 13 and the bump 14 of the magnetic layer 12.
After that, an inverse DC current is impressed on a magnetic head 3b having a magnetic gap G of a comparatively narrow gap width gb to generate a comparatively weak magnetic field M.sub.2 as shown in FIG. 15b. The magnetic field M.sub.2 is not strong enough to invert the magnetization m1 of the bump 14, but is strong enough to invert the magnetization m1 of the projection 13. So, the magnetization m1 is recorded on all of the projection 13.
Thus, the accuracy of the positioning signal depends on the accuracy of the projection patterning. Consequently, it is possible to prepare a magnetic disk having a high track density.
But in this way it is necessary to prepare two kinds of magnetic heads having a different gap width and to exchange these magnetic heads. Because the recording area is formed on both sides of the magnetic disk, it is necessary to exchange the magnetic heads two times per one magnetic disk. Therefore, it is not effective to produce the magnetic disk by this way.